If you've manually installed Oracle Java it doesn't show up in the Dash (as far as I know).

1. Open a terminal

2. Execute the following command:

   /usr/bin/jdk1.8.0_05/bin/ControlPanel
   

   Replace /usr/bin/jdk1.8.0_05 by the path of your Java installation.

If you have installed java to a particular location

cd /PATH/TO/YOUR/INSTALLATION/DIR 
./ControlPanel

You have openjdk-8-jdk installed in Ubuntu 18.04. You can use exactly the same command line utilities with OpenJDK as with Oracle JDK, but ControlPanel and jcontrol do not exist in openjdk-8-jdk in Ubuntu 18.04.

The GUI in openjdk-8-jdk in Ubuntu 18.04 is named JConsole. The JConsole graphical user interface is a monitoring tool that uses the extensive instrumentation of the Java Virtual Machine (Java VM) to provide information about the performance and resource consumption of applications running on the Java platform. Once you have connected JConsole to an application, JConsole is composed of six tabs: Overview, Memory, Threads, Classes, VM Summary and MBeans.

$ java -version
openjdk version "1.8.0_222"
OpenJDK Runtime Environment (build 1.8.0_222-8u222-b10-1ubuntu1~18.04.1-b10)
OpenJDK 64-Bit Server VM (build 25.222-b10, mixed mode)
$ which java
/usr/bin/java
$ cd /usr/bin # path to jconsole
$ jconsole

The command jconsole will open the JConsole window.

With :

sudo update-alternatives --install "/usr/bin/java" "java" "/opt/java/64/jre1.6.0_20/bin/java" 1
sudo update-alternatives --set java /opt/java/64/jre1.6.0_20/bin/java

You can set the default jre (in this case a 64 bits one)

with:

java -version

you can see the version

With about:plugins you can see which jre firefox is using

The name of the control panel is different depending on whether you are using the Oracle JDK or OpenJDK.

Search for "control panel" in the dash. If you're using the Oracle JDK, you'll see the Oracle Java 7 Plugin Control Panel. If you're using OpenJDK, you'll see the Iced Tea Web Control Panel.

try using this command in the terminal:

sudo apt install default-jre

then you can try using:

java -version

to check version